Pathogenic effect of parasites on the host

Impact of adaptation and immunity on the clinical course

The nature of the disease directly depends on the degree of the parasite's evolutionary adaptation to humans.

• Poor adaptation (zoonoses): Animal pathogens that accidentally infect humans cause acute and severe conditions. A striking example is Trypanosoma brucei rhodesiense. This is a typical zoonosis that provokes an acute form of African trypanosomiasis, frequently leading to the patient's death within the first month.
• Good adaptation (anthroponoses): Parasites specific to humans aim for long-term coexistence. Infections take a chronic course with a prolonged asymptomatic period (forming "healthy carriage"). An example is Trypanosoma brucei gambiense. Although the pathogen can infect domestic pigs, their role is minimal—the parasite has only begun adapting to them.

The course of the infestation is also determined by the host's status. Severe forms develop against the background of immunodeficiencies (e.g., HIV infection leads to the reactivation of latent toxoplasmosis with brain involvement). Genetic features are also important: carriage of thalassemia genes or glucose-6-phosphate dehydrogenase enzyme deficiency drastically reduces mortality from falciparum malaria. The resulting immunity can be persistent (in leishmaniasis) or relative non-sterile (enterobiasis, ascariasis), allowing reinfections.

Nutritional deficiency and mechanical tissue destruction

One of the basic mechanisms of host exhaustion is mundane competition for nutrients. Large worms require colossal resources. For example, Taeniarhynchus saginatus (beef tapeworm) grows by 7–10 cm daily. Parasites provoke specific avitaminoses: in diphyllobothriasis, the pathogen (Diphyllobothrium latum) absorbs vitamin B12 in huge quantities, while in cysticercosis, ascorbic acid levels drop.

In giardiasis, the pathogens line a significant area of the small intestine. This completely disrupts membrane digestion—the body ceases to normally absorb fats and fat-soluble vitamins.

Simultaneously, tissue damage occurs:

1. Cell lysis: Entamoeba histolytica secretes proteinases that dissolve the epithelium of the large intestine, forming deep ulcers.
2. Erythrocyte damage: In malaria (Plasmodium), oxidative phosphorylation in blood cells is blocked, disrupting plastic metabolism and oxygen transport.
3. Traumatization: Juvenile opisthorchiasis specimens tear bile ducts with spines and suckers. A growing cyst in echinococcosis compresses structures, causing atrophy. Schistosomiasis is accompanied by inflammation of the intestinal and bladder walls due to deposited eggs (carcinogenesis risk).
4. Destruction during migration: Larvae in nematodiases (ascariasis, ancylostomiasis) mechanically pierce lung and liver tissues.

Toxic-allergic and immunopathological reactions

Parasite metabolic products act as potent antigens. The classic pathogenesis of allergy is seen in the example of Giardia: they induce death (apoptosis) of intestinal epithelial cells and suppress the synthesis of protective immunoglobulin IgA. The epithelial barrier collapses, and low-molecular-weight allergens are unhinderedly absorbed into the bloodstream.

Toxic-allergic effects come to the forefront in many nosologies:

• In American trypanosomiasis, allergic reactions are the leading link in pathogenesis.
• In trichinosis, severe allergic myocarditis develops, which frequently causes the patient's death.
• Similar reactions are typical for fascioliasis and the migration phase of ascariasis.

The host immune response sometimes causes no less harm than the pathogen itself. Active macrophage function in malaria or visceral leishmaniasis leads to total enlargement of the liver and spleen (hepatosplenomegaly). In quartan malaria, immune complexes massively settle in kidney tissues, triggering nephrotic syndrome.

Alteration of reproduction and behavior

From the perspective of biological advantage, it is extremely beneficial for a parasite to suppress the host's reproductive function—this frees up energy resources for the pathogen's own needs. Malaria is historically associated with miscarriage, increased maternal mortality, and, in endemic areas, impotence.

Experiments on mice show that helminth metabolites (toxocaras, hymenolepids) damage the genome of spermatogenesis cells. The outcome is a reduction in generative cell activity and the mass appearance of spermatozoa with damaged (single-stranded) DNA.

To facilitate their transmission (especially when a change of hosts is required), parasites have learned to modulate behavior. Nematodes (ascarids, filariae, trichinellae) penetrate directly into the central nervous system. For example, brain tissue damage by larvae in toxocariasis provokes neurological pathology, confusion, and a sharp decline in cognitive abilities.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Carcinogenic action of parasites: how schistosome eggs provoke tumors
• Mechanisms of non-sterile immunity formation in enterobiasis and ascariasis
• Pathogenesis of falciparum malaria at the cellular level: blockade of energy metabolism
• Specificity of CNS damage by nematodes: clinical manifestations of toxocariasis
• Influence of parasitic metabolites on DNA molecules and spermatogenesis

Other topics in the section "General Parasitology Questions"

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